Currently, in one aspect, the available energy on the earth is increasingly decreasing, in another aspect, a petrifaction fuel may generate an emission of carbon oxide and sulfur oxide to pollute the air and increase a greenhouse effect, thus leading to a worse global environment and an unusual climate. Therefore, a development of pollution-free, renewable energy sources has become a primary research subject. A pollution-free solar energy is one of the best options of renewable energy. A primary task of a development of the solar cell lies in a development of a material for converting the solar energy into an electric energy with high efficiency.
It is well known that a Cu—In—Ga—Se compound which has a chalcopyrite structure may be fabricated on a soft or rigid substrate as a material for solar energy generation. Such Cu—In—Ga—Se film solar cell has advantages of high stability and low cost. The Cu—In—Ga—Se compound which is a compound semiconductor with chalcopyrite structure is a direct bandgap material, which can absorb solar lights in a wide wave length range and has a characteristic of forming a p-n junction by self-adjusting its component. Thus, the Cu—In—Ga—Se compound is considered as one of the best materials of an absorbing layer of the solar cell. For example, Cu(InGa)Se2 (referred to as CIGS) is a semiconductor material with a best light-absorbing capability up to now. A thickness of a CIGS absorbing layer is only 1-2 μm because of the high light-absorbing capability of CIGS. For a mass production, a power generation cost of CIGS is only 0.03 USD/Watt according to a rough estimation, and thus, it is of a competitive edge. It is hopeful that a cost of the solar energy generation is equal to or even lower than that of the conventional petrifaction fuel power generation. Therefore, how to fabricate the Cu—In—Ga—Se film solar cell with low cost and high efficiency has become a research focus.
A CIGS film solar cell has following two advantages: (1) A photoelectricity conversion layer is as thin as a few micrometers; (2) A bandgap (forbidden bands) of Cu—In—Ga—Se compound may be adjusted by adjusting a content of Ga (gallium). According to a prior art, a relationship between a ratio of Ga to In and the bandgap (Eg) satisfies: Eg (eV)=1.02+0.67 y+0.11 y(y−1), where y=In/(Ga+In), which represents an atom content ratio. According to a theory, the solar energy is not maximally used by a solar cell with a single bandgap, that is, photon with low energy cannot generate any electron-hole pair, while photon with high energy can only excite one electron-hole pair, and the redundant energy is converted into thermal energy unfavorable for the efficiency of the solar cell. To this end, it is desired for the solar cell to have more bandgaps to absorb more solar energy, thus improving the efficiency of the solar cell, which may be just achieved by CIGS with the characteristic of adjustable bandgap. The content of Ga may be adjusted when fabricating the CIGS film. The bandgap of the CIGS compound rises with an increase of the proportion of Ga.
Conventional methods for fabricating the CIGS film mainly comprise: (1) a selenylation method; (2) a laminating method; (3) a multi-source (such as two-source or three-source) evaporation method; (4) a sputtering method; (5) a deposition method; (6) a spray coating method; (7) a spinning coating method; (8) a vacuum heating synthesis method, etc. For the selenylation method, the laminating method, the multi-source evaporation method and the sputtering method, a sulfurization or selenylation process is required in certain step to treat the CIGS film. Sulfur atoms and selenium atoms may react with Cu—In—Ga through diffusion so as to generate a CuInGaSe compound. This process is called as sulfurization or selenylation.
The selenylation method for fabricating the film solar cell with the chalcopyrite structure has following disadvantages of a long production period, a high energy consumption, a high consumption of selenium, a toxicity of a selenium vapor, a nonuniform distribution of selenium and a gradient distribution of selenium, etc.
In addition, it is difficult for the above methods to realize an adjustment of a gradient distribution of Ga by one-step. Taking a three-step co-evaporation method developed by an NREL Lab in United States for example, an A-shaped bandgap or a V-shaped bandgap is formed by different elements participation in three steps. The process is very complicated, and a precise real time control is also required. Although the film solar cell with a high conversion efficiency may be fabricated by this method, the method is not favorable for a mass production with a low cost and big area.